1. Field of the Invention
The invention relates to a lamp driving system, more particularly to a multi-phase multi-lamp driving system.
2. Description of the Related Art
Lamps used in scanners, copy machines or liquid crystal displays serve as a backlight source. In order to enhance and obtain a uniform brightness, an increased number of lamps are needed. A lamp driving system is usually used to ensure that a current flowing through each lamp is identical such that the service life of each lamp can be extended.
Referring to FIGS. 1 and 2, conventional multi-lamp driving systems, such as that disclosed in U.S. Pat. No. 6,534,934, are known in the art. However, in the conventional multi-lamp system of FIG. 1, when the impedance of a capacitor (C1) and a lamp 91 connected in series does not match the impedance of another capacitor (C2) and a lamp 91 connected in series, the currents flowing through the lamps 91, 92 are different from each other such that a uniform brightness of the lamps 91, 92 cannot be ensured. Furthermore, the one of the two lamps 91, 92 supplied with the larger current has a decreased service life. In the conventional multi-lamp system of FIG. 2, the impedances of the loads (Za, Zb, Zc) can be adjusted such that the currents flowing through the lamps 91, 92 are identical. However, the conventional multi-lamp driving system of FIG. 2 is only suitable for two lamps.
FIG. 3 illustrates another multi-lamp driving system for driving a plurality of lamps 91, 92, 93, 94 (more than two lamps). The system is formed by connecting in parallel a number (i.e., two) of the conventional multi-lamp driving systems of FIG. 2. However, as shown in FIG. 4, a current ripple (Ir1) flowing through the inverter 81 and a current ripple (Ir2) flowing through the inverter 82 have the same amplitude and the same phase such that a total current ripple (IM), which results from the constructive interference of the current ripples (Ir1, Ir2), generated by the conventional multi-lamp driving system of FIG. 3 is increased. Electromagnetic interference occurs as a consequence.